Means for reducing thermal gradients in the hot-pressing of synthetic mica



' Aug. 13, 1963 INVENTORS WILLIAM MC NEILL JOSEPH E. CHROSTOWSKI AT TO R NEY Q MEANS FOR REDUCING THERMAL GRADIENTS IN THE HOT PRESSING F SYNTHETIC MICA William McNeil], and Joseph E. Chrostowski, Philadelphia, Pa., assignors to the United States of America as represented by the Secretary of the Army Filed May 5, 1961, Ser. No. 108,206

1 Claim. (Cl. 100-93) (Granted under Title 35, US. Code (1952), see. 26-6) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to improved hot-pressing apparatus for synthetic mica and more particularly concerns the uniform recrystallization of synthetic mica by eliminating or reducing thermal gradients involved in their'hotpressing. 1

In copending applications by McNeill et al., Method of Making Lamellar Sheets of- Fluor-phlogopite Mica, Serial No. 3 8,673, filed June 24, 1960, and Apparatus for Electrophoretic Deposition of Lamellar Fluorphlogopite Mica Sheets, Serial No. 108,209, filed May 5, .1961,- both assigned to the assignee hereof, a method and apparatus for making lamellar polycrystalline sheets of synthetic mica by electrophoretic deposition were disclosed. The larnel- *lar deposits resulting therefrom represent approximately 3,100,436 Patented ,Aug. 13, 1 963 Ice jects and advantages thereof will be apparent from consider-ation of the following description and single drawing illustrating a vertical section of the improved hot-pressing apparatus of the invention immediately prior to pressing.

For purposes of clarity of illustration, the press equipment, induction heating apparatus, methods of cooling and other standard pieces of equipment normally used in conjunction with hot-pressing are not shown. Such equipment is employed in connection with the present invention, but forms no part thereof. The apparatus hereinafter described is adapted to be attached to and to be utilized in connection with any standard press machine and induction heater. For example, a 60 ton press manufactored and sold by Manley Company of York, Pennsyivania, alfords the necessary mechanism for effecting sufficient pressure on the mica to be pressed. Similarly, the induction heater may be a Lepel High Frequency Induction Heater, Lepel, Inc., Woodside, New York.

Referring now to the drawing, thereis shown an electrophoretic deposit of polycrystalline mica 10 for form- The mica will preferably be approximately /z-3 inches in diameter, but

onealf the theoretical density of pure natural mica, and

in order for such material to find use in various electrical applications, the deposits must subsequently be converted, such, for example, as by hot-pressing, into flexible and impermeable sheets of suiiicient dimensions. The hotpressing of mica, however, is unique, in that its study positively indicates a considerable weight loss from the decomposition therof at temperatures upwardly of about 900 C., and due principally to the volatilizati-on of flue rides contained therein. At temperatures approaching 1300" C. or higher, wherein the mica becomes recrystallized, the weight loss thereof is very rapid.

"The hot-pressing of mica by an oscillating high-frequency electric current inherently permits very rapid thermal cy-clings which is highly desirable in its application to mica, since this material is subject to excessive surface decomposition when held at its recrystallization temperature for long periods. A principal disadvantage of highfrequency induction heating in the hot-pressing of mica however, resides in the production of undesirable thermal gradients in the normally employed circular graphite blocks adjacent their outer rims which results in a nonuniform mica.

We have overcome .this problem by making the graphite blocks of prescribed oversize dimensions to provide an inner area of near-uniform temperature. Further, the pressure transmitting members contacting the graphite blocks comprise a low heat transfer coefficientmaterial, such as m-ullite, to prevent the introduction of additional gradients at this inner area.

It is a principal object of this invention to provide an improved hot-pressing apparatus wherein depositions of polycrystalline lamellar sheets of synthetic mica may be successfully converted to generally transparent crystalline mica sheets of sutlicient dimensions which are flexible and impermeable.

The exact nature of this invention as well as other 0b may be of any shape, although a generally rectangular or circular piece will ordinarily be used. A pair of molybdenurn sheet discs 11-11 are remova-bly arranged in sandwiching relation to the mica 10. While molybdenum is preferred for this purpose, platinum and palladium have been found to Work satisfactorily. A graphite block 12, suitably cylindrical in design although not specifically limited thereto, is vertically positioned against the exposed side of each of the molybdenum discs.

7 In accordance with the invention, the graphite blocks 12-12 will have a greater diameter than the molybdenum discs 11-11, which, in turn, will be slightly larger than the mica piece. We have found that uniform recrystallization of the mica willoccur even though theternperature of the graphite blocks is raised to about 1300 F. in less than. 30 minutes providing the mica piece is overlapped therearound by at least about 1 inch of the blocks. Following customary practice, the graphite blocks, molybdenum discs and mica piece will have therearound an air-tight shield, not shown, to provide for passage of a suitable inert gas therethrough. The molybdenum discs are chemically inert in the presence of these gases and not subject to attack by volatile fluoride gases given off by the mica as would be the graphite surfaces in the absence of the molybdenum discs. Refractory punches 13 '13, suitably of mullite, and positioned as shown, serve wiched by the molybdenum discs. The sandwiched mica will then be centrally positioned between the graphite blocks and the entire assembly axially aligned to rest on the lower mullite punch. The upper punch will be brought into contact with the upper graphite block and the shield positioned. The chamber enclosed by the shield will next be flushed with gas after which the heating process may be started.

We claim: I

Apparatus :for use in inductively hot-pressing a lamellar polycrystalline sheet of synthetic mica comprising a pair of discs made of a metal selected from the group consisting of molybdenum, platinum and palladium for receiv ing said mica therebetween, a graphite block disposed vertically outwardly of each disc and axially aligned therewith, said graphite blocks being of sufficient dimensions to overlap said mica thereamund by at least about 1 inch,

, 4 inductor coils surrounding said graphite blocks, and means to transmit pressure to the mica to be pressed;

References Cited in the file of this patent UNITED STATES PATENTS McNeill et al. May 10, 1960 

